Voltage drop calc

[seifdogg]

Concerning voltage drop for long distances, should one always begin the circuit with the conductor with the most ampacity? I'll lay out my specifics now. 1350' total circuit length. 92A motor load at end of circuit. 21A motor load 100' from supply. Since most of the resistance is going to be in the 1250' run between the two loads, can I undersize the 100' from supply to first motor?

Note: I erred on the side of caution and ran parallel #4/0 AL to first load, then #350 to second. Next time?


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[iwire]

I'll lay out my specifics now. 1350' total circuit length. 92A motor load at end of circuit. 21A motor load 100' from supply.

You have one 1350' circuit powering 21 amp and 92 amp motors 1250' apart? :huh:

That seems like a really odd ball circuit so I think it would be worth running the calculations on a number of ways to find the best compromise.

With a 21 amp load only 100' from the source I would really be considering running that on a separate set of conductors and dealing with the 92 amp load on its own.

 

[seifdogg]

I like this suggestion, however, the first motor is on the way to the second motor, and made the most sense to consolidate it on one conductor run.


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[Dennis Alwon]

It does not matter if you start or end with the largest capacity wire. That said one normally one sees the larger capacity conductor first

 

[iwire]

It does not matter if you start or end with the largest capacity wire.

That is true when there is only one load, when you have multiple loads that can change.

But the OPs is a really odd situation, two loads, far apart, with one close to the source and very different currents.

 

[kwired]

The first segment has both loads on it. Any voltage drop in the first segment needs to be factored in to voltage at the end of the second segment.

If you figured that you have 2 volt drop on first segment and a 4 volt drop on the second segment then you have a net drop of 6 volts at the 2nd load. If you increased the size of first segment so it only has a 1 volt drop then that leaves you with only 5 volts net drop at the 2nd load.

Seems pretty straight forward but some seem to forget you don't start over with original full voltage at each new segment you start with whatever drop has already occurred up to that point.

 

[Sahib]

It is possible to convert the two motor loads into an equivalent single load acting at a specific distance from the source. Then the calculation for the voltage drop may be carried out for the equivalent load and wire size selected which is applicable for the two motor loads. So proceed this way. To find the distance at which equivalent load[=sum of two motor loads] acts, multiply each load by respective distance from source and sum up.Divide the sum by the equivalent load. The result is the required distance. Now as you know equivalent load and its distance from source, you could easily calculate size of wire required for a given voltage drop.